General digital thermometer question Thermistor

[Neosec]

I'm in the process of doing a thermostat project and have been looking over Digital Thermometer schematics that use thermistors. I've seen some with elaborate dual op-amp circuits and others that feed strait to a PIC's ADC VIA voltage divider. Is a buffer of some kind recommended? If so why? I want to use this in a farm's out-building when I'm done so a semi harsh environment is likely.

[mister_e]

It's down to the Thermistor specs... and MAX PIC ADC impedance...

[skimask]

I'm in the process of doing a thermostat project and have been looking over Digital Thermometer schematics that use thermistors.

Or just make it easy on yourself and use a DS1820.

[Neosec]

I considered a DS1820 but was concerned about the 1-wire network immunity to noise... I want to have at least 5 temperature sensors 3 indoor, two of which will be up to 200 feet from the PIC probably running along side Romex 120v AC lines. An outdoor sensor and a sensor in a thermowell in a boiler ~180 Deg.F. Thermistors are industry standard in a boiler so I didn't want to reinvent the wheel. And as for the longer runs I just figured the thermistor lines could be filtered with some caps since they're DC so noise would be less of a problem. I guess adding a buffer will provide a more robust signal. Thanks... I'll look into it a bit more now that I know what to look for.

[Charles Linquis]

I have successfully used the Microchip MPC9701. Since it has an analog output, you can run the signal 50' or more. Just use a low-pass filter (2K in series, .1uF to GND) with the A/D input and average 16 samples with a 1mSec delay between samples (so that you average out over one complete cycle of 60Hz). The output is linear with temperature (unlike thermistors), and doesn't need calibration. Cheap, too.

[mackrackit]

LM34 or LM35 will work too. A cap from signal to ground for longer distances and good to go.
Like the Microchip MPC9701 these are also linear. Your harsh environment is not a problem. Good for around 300 F.

Industry standard? I use the LM34 in thermo wells at the plant where I work. One has an average temp 250 F. One degree resolution, no problems. We must be Non- standard. :)

Lots of options.